This invention relates to defect-free, ultrahigh flux asymmetric membranes having ultrathin selective layers for separating fluids, especially gases. In another aspect, the invention relates to a novel dry/wet phase separation process for preparing integrally-skinned asymmetric membranes having simultaneously improved productivity and selectivity. In yet another aspect, the invention relates to a process for separating one gas from a gaseous mixture by selective permeation of the gas through an asymmetric membrane with an ultrathin defect-free selective surface layer of less than 0.2 .mu.m comprised of glassy polymers.
A variety of applications in the chemical processing industries, including production of blanketing gases, recovery of components from refinery and chemical synthesis streams and treatment of product gases from enhanced oil recovery operations would benefit from improved gas separation processes. Separating at least one selected gas from a gaseous mixture using processes based on membranes with the capability for selectively permeating one or more gases can provide a product enriched in the one or more desired gases relative to the feed composition. For commercial viability, the membranes in such processes must have adequately high productivity coupled with a sufficiently high selectivity.
Loeb and Sourirajan (hereafter "LS"), in U.S. Pat. No. 3,133,132 disclosed a method for the preparation of cellulose acetate membranes for desalination of water. This patent teaches a method of membrane formation in which a thin layer of cellulose acetate solution is cast on a suitable support followed by solvent evaporation and subsequent quenching in cold water to produce an asymmetric membrane comprised of a thin selective surface layer supported by a microporous substructure.
This asymmetric structure provides extremely high selectivity in reverse osmosis operations because the thin selective layer is capable of discriminating between the water and dissolved salts to almost the same degree as is possible using a much thicker dense film of the same material. The productivities of asymmetric membranes are inversely proportional to the apparent thickness of the thin selective layer. By virtue of its much thinner selective layer, and hence much lower resistance to passage of permeant water, these asymmetric structures also offer tremendously higher productivities.
However, it has been found that asymmetric cellulose acetate membranes have not been used without problems in reverse osmosis applications, i.e., lack of temperature, chemical and microorganism resistance. Therefore attention has been drawn to formation of asymmetric membranes from materials other than cellulose-based polymers to provide stronger structural properties and increased chemical resistance. The development of integrally-skinned asymmetric membranes based on the LS process using these alternate materials has met with significant difficulties in terms of achieving adequate selectivity and high permeation rates.
Typically the noncellulose-based polymer membranes made by the LS process yield structures which have either microporous skins and unacceptably low selectivities or overly thick selective layers which produce unacceptably low productivities. Such asymmetric membranes therefore, have often failed to satisfy requirements in liquid separation operations such as reverse osmosis.
More recently attempts to produce membranes for use in gas separations have been emphasized. Attempts have been made to draw on knowledge developed in the liquid-liquid separation membrane field. An additional problem that arises in utilizing LS membranes quenched in aqueous nonsolvents for gas separation applications is their tendency to collapse upon drying the subtle morphology needed to produce selectivity and productivity. This problem has been overcome by solvent exchange techniques to control the interfacial tension acting in the porous structure. However, current solvent exchange techniques are multistep processes, making the overall, conventional LS asymmetric membrane preparation procedure rather complicated and expensive. The LS technique, even when combined with modified casting protocols has proved to be only marginally acceptable for the more demanding requirements of perfection of the selective skin in the gas separation case.
Permeation occurs by a so-called solution/diffusion mechanism and pore flow through the defects of the skin layer. The solution-diffusion mechanism involves interaction of the gas in the external feed stream with the upstream membrane face to produce a concentration gradient across the selective skin to drive the permeation process.
While the presence of some small pores in the membrane can be tolerated in liquid separations such as desalination, the exceedingly small dimensions of gas molecules in gas separation systems, means that pores even as small as 5.ANG. cause unacceptable losses in selectivity if present at more than a few ppm in area fraction (H. M. S. Henis, M. K. Tripodi, "Composite Hollow Fiber Membranes For Gas Separations: The Resistance Model Approach" J. of Membr. Sc. 8,233-246 (1981).) Therefore, separation is significantly affected by the size and number of defects in the skin layer of asymmetric membranes. Methods of post-treatment to eliminate surface defects of asymmetric membranes include annealing and gas, vapor or solvent treatments. However, the membrane productivity usually is decreased by these treatments (see for example U.S. Pat. Nos. 4,486,202 and 4,472,175).
The low selectivities caused by the presence of defects in gas separation membranes was also addressed by an earlier coating technique introduced by Henis and Tripodi, U.S. Pat. No. 4,230,463. The multi-component membrane produced by this coating process typically comprises a silicone rubber coating on an asymmetric membrane made of a glassy polymer. Such multicomponent membranes allow fabrication from a wide variety of materials. The asymmetric membranes used in this embodiment were spun from solvent, or solvent/nonsolvent dopes of glassy polymers, especially aromatic polysulfone, and coagulated in water. Typically, selective layers on these membranes were on the order to 1500 to 2500.ANG.. Elimination of Knudsen and viscous flow through defects in the selective layer using this technique resulted in significant improvements in selectivity at a cost of productivity. The productivity is determined by the series diffusional resistance contributed by the rubber coating and the selective glassy polymer layer. A tradeoff between productivity and selectivity, therefore, is inevitable in applying this approach.
To make membrane-based gas separation useful in industrial applications, a skin thickness of less than 0.2 microns is generally required. Two casting variables favor the formation of essentially defect-free membranes made by conventional casting procedures: First, the polymer concentration at the nascent membrane/coagulant interface should be as high as possible. Therefore casting dopes usually contain 20 to 30% polymer. A further approach to increase the polymer concentration is to partially evaporate the solvent of the casting dope. Second, a relatively dense skin layer can be obtained by controlled (slow) precipitation. Although both approaches usually lead to essentially defect-free gas separation membranes, the resulting selective layer thickness is commonly in the order of 0.1 to 6.0 micron (U.S. Pat. No. 4,666,644). Thus, permeation rates of most conventional integral-asymmetric membranes are relatively low.
Several approaches for producing high productivity membranes from selected polymers (polyethersulfone and polyetherimide) having selectivities close to those of the intrinsic selectivities of these glassy polymers have been described in U.S. Pat. No. 4,746,333 and German Patent No. DE 3420373 C2. These techniques, however, involve either modifications of the precipitation liquid to include a polymeric blocking agent or a subsequent post-treatment to repair defects associated with the formation of very thin selective layers on the order of 500.ANG. to 2000.ANG. (U.S. Pat. No. 4,746,333 and German Patent Application No. P 3615649.3-44.).
Kesting et al. (EPA No. 0257012 A2) have proposed a concept based on formation of a graded density skin to produce high productivity membranes by incorporation of high levels of frozen free volume in the selective layer of asymmetric membranes used in gas separation. It is claimed that the increase in free volume of the membrane skin is indicated by an increase of the glass transition temperature of the membrane sample compared to that of the bulk polymer. The membranes are formed by immersion in water of a casting solution comprised of a Lewis-acid-base complex solvent system close to the point of incipient gelation. Without using the coating technique of Henis and Tripodi cited above, these membranes have unacceptably low selectivities due to their much higher surface porosities which accompany their apparently thinner skins. Upon coating, these membranes show significant increases in selectivity with small reductions in productivity. As opposed to the membranes described by Henis and Tripodi, the new membrane structure is more productive; however, it represents a step backward in terms of generation of a truly defect free glassy polymer surface layer.